Revert "Move wm/core to wm namespace."

This reverts commit 067d7fe616a07011b9e2d2609fc65af8db6a7dfb.

Broke the build:
http://build.chromium.org/p/chromium.mac/builders/Mac%20Builder%20%28dbg%29/builds/63598/steps/compile/logs/stdio

BUG=
TBR=ben

Review URL: https://codereview.chromium.org/198283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256686 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256686